The use of ceramic compositions to form components for the electronics industry is well known. One application for a ceramic composition with a low dielectric constant (K) involves patch antennas. A low dielectric constant material may be required in order to achieve a wider bandwidth for certain antenna applications. Moreover, significant cost savings and processing advantages may be achieved when a ceramic composition is used to form a patch antenna using a low cost forming method such as pressing or tape casting.
Another application for a low dielectric constant ceramic composition involves substrates for multilayer ceramic packages. Multilayer ceramic circuits have been used for many years to increase circuit functionality per unit of area. Low dielectric constant materials are often required for ceramic substrates used in high speed applications, such as IC interconnects, for example.
A ceramic composition with a low dielectric constant must also possess other desirable properties in order to find wide scale usage in electronic applications. For example, if the required sintering or firing temperature of the ceramic composition is too high, then only electrically functional materials with very high melting temperatures, such as tungsten and molybdenum, may be used to form the conductors and transmission lines attached thereto. Thus, cofirability is an important material property. Tungsten and molybdenum tend to have poor electrical properties and, in particular, poor conductivity properties which render them unsatisfactory for many high-speed, complex radio frequency ("RF") circuitry applications. Moreover, such compositions are often fired at a temperature of approximately 1600.degree. C. in a hydrogen atmosphere for extended periods of times In addition to the high processing costs, materials such as tungsten and molybdenum are also very expensive.
On the other hand, if the ceramic composition has a lower firing temperature, it may be cofired with a wide variety of highly electrical functional materials such as silver, palladium/silver, gold, platinum, nickel and copper. Of course, as the sintering temperatures decrease, processing costs during manufacturing decrease accordingly.
There are several other material properties of interest for use in multilayer RF devices and the like. Low loss, i.e., a high Q (small loss tangent) material is a property which helps the overall performance of the device. Electrical Q is defined as the inverse of the loss tangent. The loss tangent is defined as the ratio of the imaginary part of the complex dielectric constant of a material to its real part. An electrical Q value greater than 900 is most desirable. Another important property is the coefficient of resonant frequency ("T.sub.f ") which is the measure of how much the resonant frequency of a filter made using a material shifts with a change in temperature. A low T.sub.f value, measured in ppm/.degree.C., is also very desirable. From a materials perspective, the ability to tape cast a composition using a doctor blade technique in conjunction with traditional tape casting processes is extremely advantageous in the formation of green tapes for multilayer and other applications. Moreover, if a composition is sufficiently flexible such that it may be adapted to various different forming methods, such as both dry pressing and tape casting, this would also be highly desirable.
The use of cordierite to provide a low dielectric constant (K) is well known in the prior art. U.S. Pat. No. 5,206,190 issued to Jean discloses a dielectric composition containing cordierite and glass. U.S. Pat. No.5,532,194 issued to Kawashima discloses a cordierite glass-ceramic and method for manufacturing the same. A novel composition may be achieved when a celsian phase is created when a glassy precursor material is combined with a high purity cordierite composition such that the celsian phase is formed at low temperatures and offers desirable electrical properties such as high Q and low K.
Accordingly, there exists a need to provide a multilayer ceramic material and methods of making those materials and/or multilayer substrates that demonstrate the characteristics necessary for economical use in RF device applications while avoiding the limitations inherent in the prior art. Moreover, the ceramic material must be easily and repeatably fabricated, and amenable to manufacturing methods that produce materials with consistent electrical and mechanical properties.